Monolithic integrated circuit image sensors are known in the art. Examples include Charge-Coupled Devices (CCDs) and CMOS image sensors. Refer, for example, to Janesick, J. R., Scientific Charge-Coupled Devices (SPIE Press 2001); Holst, G. C., CCD Arrays, Cameras and Displays (SPIE Press 1996); and Moini, A., Vision Chips (Kluwer Academic Publishers 1999). Digital image processing algorithms are known in the art. Refer, for example, to Gonzales, R. C. and R. E. Woods, Digital Image Processing (Addision Wesley 1992).
Image sensors such as CMOS and CCD image capture devices are known. Such devices are typically designed to work in conjunction with an external framestore and a host processor.
One of the issues that arises when such image sensors are used in systems with a host processor is that the link between the image sensor and the host processor must support the relatively high read-out data rate of the image sensor.
It is an object of the invention to provide alternative architectures that overcome some of the problems associated with direct coupling between the image sensor and the host processor.
Active pixel cells have a storage node which stores a charge. During an integration period, the stored charge is modified from an initial level. Once the integration is completed, the amount of charge determines an output voltage, which can be used to drive an output circuit. The output of the output circuit is controlled by the voltage, and hence the charge, of the storage node.
In conventional pixel cells, switching into and out of the integration period causes one or more voltage drops at the storage node due to various capacitances in the circuit. This reduces the potential dynamic range of the pixel cell.
It would be desirable to provide a pixel cell that overcomes or at least reduces the impact of these voltage drops without requiring complicated additional circuitry. It would be even more desirable if a fill factor of such a pixel cell was not substantially different to that of prior art pixel cells.